Abstract
Multi-specific synchronous spawning risks both sperm limitation, which reduces fertilization success, and hybridization with other species. If available sperm of conspecifics are limited, hybridization with heterospecific sperm could be an alternative. Some species of the reef-building coral Acropora produce hybrid offspring in vitro, and therefore hybridization between such species does sometimes occur in nature. Here, we report that the interbreeding species Acropora florida and A. intermedia preferentially bred with conspecifics at optimal gamete concentrations (10(6) cells ml(-1)), but when sperm concentration was low (10(4) cells ml(-1)), A florida eggs displayed an increased incidence of fertilization by sperm of A intermedia However, A intermedia eggs never crossed with heterospecific sperm, regardless of gamete concentrations. It appears that A florida eggs conditionally hybridize with heterospecific sperm; in nature, this would allow A florida to cross with later-spawning species such as A intermedia These results indicate that hybridization between some Acropora species could occur in nature according to the number of available sperm, and the choice of heterospecific sperm for fertilization could be one of the fertilization strategies in the sperm-limited condition.